Various thermal insulation techniques are described for example in the following documents: FR-98/16,791, JP-2,176,299, or WP-97/47,174.
Thermal insulation can be obtained with various processes. Cellular or woolly porous solid materials that block the convection of low thermal conductivity gas are used onshore or in shallow water. The compressibility of these porous materials does not allow this technique to be implemented at relatively great depths.
Another well-known technique consists in coating the pipe with a first layer of a paraffin-imbibed porous material for example, whose thermal insulation coefficient is lower than that obtained with the gas trapping technique reminded above, and with a second layer of a refractory material that reinforces the effect of the first layer. However, such a solution cannot be used in water.
Other solutions are more suitable for use at great depths of immersion. The following materials can be used for example:                coatings made of quasi-incompressible massive polymeric materials based on polyurethane, polyethylene, polypropylene, etc., which however exhibit a rather average thermal conductivity, insufficient to prevent drawbacks in case of production stops, or        coatings made of syntactic materials consisting of hollow balls containing a gas and withstanding the outside pressure, embedded in binders such as concrete, an epoxy resin, etc., whose conductivity is lower than that of compact materials, but which are much more expensive.        
The pipe carrying the fluids can also be protected by means of an external pipe withstanding the hydrostatic pressure. A low thermal conductivity heat insulator left at atmospheric pressure or placed under vacuum, with partitions at regular intervals for safety reasons, is for example interposed in the annular space between the pipes.
It is also well-known to interpose, between the pipe and a deformable protective covering, an absorbent matrix sheathing the pipe, impregnated with a quasi-incompressible liquid/solid phase-change material at a fusion temperature above that of the surrounding medium and lower than that of the fluids circulating in the pipe.
Phase-change materials (PCM) behave like heat accumulators. They reversibly release this energy during their solidification (crystallization) or absorb this energy during fusion. These materials can therefore allow to increase the duration of production stops without pipe clogging risks through premature cooling of the pipe content. Known examples of phase-change materials are the chemical compouns of the CnH2n+2 alkane series such as, for example, paraffins (C12 to C60), which offer a good compromise between the thermal and thermodynamic properties (fusion temperature, latent heat of fusion, thermal conductivity, heat-capacity rate) and the cost. These compounds are thermally stable in the working temperature range considered, and they are compatible with a use in a marine environment on account of their water insolubility and of their very low toxicity level. They are therefore well suited for thermal insulation of deep-sea pipes for example.
The state change temperature of these phase-change materials is linked with the number of carbon atoms of the hydrocarbon chain and it can therefore be adapted for a particular application. In order to obtain a phase change around 30° C., a mixture of mainly C18 paraffins can for example be used, such as Limpar 18-20 marketed by the CONDEA Augusta S.p.A. company.
It is also possible to consider using waxes, normal paraffins, very weakly branched (1 or 2 branches) long chained isoparaffins (C30–C40), long chained branched alkylcycloalkanes or long chained branched alkylaromatics, also weakly branched, fatty alcohols or fatty acids.
Above their fusion temperature Tf, phase-change materials (PCM) are in the liquid state and their viscosity is low. In order to overcome this drawback, which can be particularly disadvantageous in certain applications, notably for the manufacture of double-walled vessels or of energy storage drums, it is well-known to add a thickening agent such as silica to solidify them and to prevent leaks.
Another drawback of phase-change materials (PCM) is that their viscous liquid state favours convection heat losses.